As an example of a conventional TFT-LCD backlight, a cold cathode fluorescent lamp 100 is illustrated in FIG. 7. In the fluorescent lamp 100, a glass tube 120, which includes a pair of cup-shaped metal electrodes 110 inserted thereinto, is melt-sealed at both ends thereof using respective lead-in wires 130 having the same thermal expansion coefficient as that of the glass tube. Upon the fabrication of the lamp, even though the lamp is exhausted to a high vacuum level, primary electrons, naturally occurring due to cosmic rays, are present therein. In the lamp fabrication process, after evacuation, the lamp is filled with Ne—Ar gas 150 at a pressure of 50 torr or more. When alternating current of high voltage is applied to both ends of the lamp, the primary electrons are accelerated by the electrical field, thus ionizing the gas 150. When such ionization continues, spark plasma, in which cations 160 and negative electrons 140 coexist, is formed. The formed cations and electrons collide with both of the metal electrodes 110, and thus are neutralized. In this case, secondary electrons are generated from the metal electrodes due to collisions, thereby making continuous discharge possible. Thus, the generation of secondary electrons is regarded as an important factor in realizing continuous light emission. As the emission of secondary electrons is facilitated, high brightness is maintained.
When the electrons in the plasma collide with neutral mercury atoms 170, the mercury atoms 170 are excited. When the excited mercury atoms 170 return to a ground state, UV light 180 is emitted. The emitted UV light 180 is incident on a phosphor 190 applied on the inner wall of the lamp tube, and is thus converted into visible light 181. As such, the electrons 140 or cations 160, colliding with the metal electrodes, create sputtering at the electrodes. The metal electrode component, scattered through sputtering, is bound to mercury, thus forming a compound. When this compound is deposited around the electrodes, blackening occurs, which results in a decreased lifetime. The problem of the decreased lifetime is a big problem with the cold cathode fluorescent lamp.
In order to overcome this problem, there have been proposed 1) a method of decreasing the discharge initiation voltage using penning effects depending on the excitation and ionization of the neon-argon gas 150 loaded into the lamp, to thus reduce the impulse of electrons 140 or cations 160 colliding with the metal electrodes 110, thereby diminishing the generation of sputtering, and 2) a method of decreasing the discharge initiation voltage by dropping the gas pressure as low as possible.
However, in the cases of 1) and 2), when the discharge initiation voltage is low, the kinetic energy of the cations 160 or electrons 140 colliding with the metal electrodes 110 is decreased, undesirably reducing the emission of secondary electrons from the metal electrodes 110, resulting in decreased brightness.
In order to overcome this problem, there has been proposed 3) a method of selectively using a material having a low work function as that of the material for the metal electrode 110, to thus facilitate the supply of electrons from the metal electrode 110. However, in the case of 3), the fabrication cost is raised because the price of the metal electrodes 110 is high. Further, there is a problem in which expensive borosilicate glass must be used in order to adjust the thermal expansion coefficients of the glass tube 120 and the lead-in wire 130. Whereas the cold cathode fluorescent lamp 100 has low tube resistance, the resistance component thereof is dominantly large, and thus one transformer is responsible for driving only one lamp, inevitably incurring an increase in the total fabrication cost. Moreover, as the diameter of the tube is increased, brightness is drastically decreased, so that the lamp is required to be mechanically strong. Ultimately, it is difficult to apply the above lamp to large-sized TVs requiring a lamp having a large diameter (tube diameter: 4 mm or larger) as a backlight.
In order to partially solve the problem, there has been developed an external electrode fluorescent lamp, in which the outer surfaces of both ends of a glass tube are coated with a conductor, or are brought into close contact with a metal cap, to thus enable parallel driving using a capacitance component of glass, which is illustrated in FIG. 8.
In the external electrode fluorescent lamp 200 of FIG. 8, phosphor is applied on the inner surface of a glass tube 210, both ends of which are sealed. The inner space of the glass tube 210 is filled with a charge gas mixture comprising inert gas, such as argon (Ar) or neon (Ne), and mercury (Hg) gas. Further, an external electrode having one of various shapes, coated with a conductive layer 221 including silver or carbon, is provided at each of both ends of the glass tube 210, and is fitted with a metal cap 220.
As for the external electrode fluorescent lamp 200, when high voltage alternating current (AC) is applied to the conductive layer 221, both ends of the glass tube 210, in contact with the external electrodes 220, play a role as a dielectric material, leading to a strong induced electric field. More specifically, when the polarity of the voltage applied to the external electrode is (+), electrons accumulate in the glass tube coated with the conductive layer. On the other hand, when the polarity thereof is (−), cations accumulate. The wall charges accumulated through continuous polarity conversion using the alternating current electric field reciprocate between the opposite ends of the glass tube. As such, while the wall charges collide with the mercury gas that is supplied together with the inert gas, the excited light emission of the mercury gas is induced. Then, UV light, produced during this emission, excites the phosphor applied on the inner wall of the glass tube, thereby causing it to emit visible light.
Further, the UV light thus radiated excites the phosphor applied on the inner wall of the glass tube 210. Accordingly, when light is emitted from the inner space of the glass tube 210, light is radiated externally.
In the conventional external electrode fluorescent lamp 210, as the areas of both ends of the glass tube 210, functioning as the dielectric material, to be coated with the conductive layer 221, are enlarged, the magnitude of wall charges is increased, whereby the brightness of the lamp may be increased to some degrees. However, there is a limitation in the ability to extend the conductive layer 221 in a longitudinal direction. In the case where the conductive layer 221 extends in a longitudinal direction, the area in which light is externally radiated is decreased, undesirably reducing the emission efficiency.